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REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 386
New Horizons for Use of Cellulose-Based Materials to Adsorb Pollutants from Aqueous Solutions
Martin A. Hubbe
This article reviews recent research related to biosorption – the use of plant-derived materials to remove various pollutants from aqueous systems. Emphasis is placed on biosorption studies dealing with the removal of heavy metal ions, dyes, and spilled oil from water. Much progress already had been achieve in understanding the factors that affect adsorption capacities, rates of uptake, and possible release back into the water. It has been shown that the performance of cellulose-based sorbent materials often can be improved by physical of chemical modification of the sorbent. There is a critical need for research related to strategies for dealing with the adsorbent materials after their use. In addition to regeneration and re-use of sorbent materials, attention also needs to be paid to the incineration of contaminated sorbents, as well as the biodegradation of sorbent material after uptake of various pollutants.
Keywords: Biosorption; Heavy metal ions; Textile dyes; Spilled oil; Water treatment
Contact information: Department of Forest Biomaterials, North Carolina State University, Campus Box
8005, Raleigh, NC, USA 27695-8005; [email protected]
INTRODUCTION
The term “biosorption” has been widely used to describe systems in which plant-
derived materials are used to remove various pollutants from aqueous systems. This
article reviews research related to the adsorption of three important categories of
pollutant: heavy metals, dyes, and spilled oil. In each case there has been an acceleration
of interest within the scientific community, resulting in numerous publications. Also, in
each case there is evidence that the effectiveness of water cleanup can be enhanced by
physical or chemical modifications of the cellulosic material in many cases.
This article will make frequent references to a series of more narrowly focused
review articles dealing with biosorption of heavy metals, dyes, and oils, respectively
(Hubbe et al. 2011, 2012, 2013). Each of the cited articles includes extensive tabulation
of the main findings presented in published articles up to that point in time. In addition,
other recently published primary literature will be cited, showing advances in the state of
knowledge concerning biosorption of metals, dyes, and spilled oils.
Two main approaches used in biosorption for the removal of contaminants from
water are illustrated in Fig. 1. In batch operations it is usual for the sorbent material to be
added to an agitated volume of the solution to be treated. In a subsequent operation the
mixture is filtered to remove the sorbent, with the goal of producing decontaminated
water. In flow-through or “packed bed” systems the sorbent material is held in place,
while the water to be decontaminated is passed through it. The process is continued until
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 387
the concentration of contaminant in the emerging stream begins to rise. At that point the
sorbent material is either replaced or regenerated.
Contaminated
water
Biosorbent
Spent sorbent
Decontaminated water
BATCH
SYSTEM
CONTINUOUS
SYSTEM
Contaminated
water
Decontaminated
water(periodic regenera-tion or replacement)
Packed bed
Separation
Fig. 1. Schematic representations of two main schemes used for biosorption of pollutants from contaminated waters
To a large degree, the step of getting the target pollutants to adsorb onto the
surfaces of cellulosic materials – and thus get them to be removed from the water – has
already been solved through use of a wide range of plant-derived byproducts, either as-
received or in chemically modified form. Modification strategies, including not only
chemical modification but also mechanical treatments, torrefaction, pyrolysis, and
activation of carbonaceous byproducts from cellulosic materials have been shown to
increase the sorption efficiencies in numerous cases (Babel and Kumiawan 2003; Dias et
al. 2007; O’Connell et al. 2008; Hubbe et al. 2012). However, the fact remains that
biosorption technology is still very far from being fully implemented at an industrial or
municipal level. Clearly there are some practical as well as theoretical barriers that are
inhibiting the deployment of systems that have appeared highly promising according to
the results of many academic studies.
KEY CHALLENGES FACING BIOSORPTION TECHNOLOGIES
The toxic nature of various water-soluble metal species, various dyes, and various
spilled oils have been shown in past work (Aguilera et al. 2010; DeLaune and Wright
2011). Despite efforts to minimize such releases, the accelerating development of
modern technology has tended to increase the amounts of such toxins in the worlds
waterways and supplies of fresh water intended for various applications.
When academic scientists are looking for suitable topics by means of which to
train the next generation of researchers, biosorption studies can appear to offer many
attractive features. In the first place, idealistic students appreciate the opportunity to
devote their efforts to the resolution of important environmental problems. Secondly,
such efforts can be focused on whatever types of industrial pollutants that happen to be
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 388
prevalent in a given region. Likewise, the work can be presented as a way to make
advantageous use of selected regionally available cellulosic byproducts that somebody
has regarded as “waste”. In the course of modifying the cellulosic substrate, students can
learn some advanced chemical strategies, various statistical analysis methods, the use of
rate expressions, and concepts related to adsorption isotherms. As a result of this
combination of favorable circumstances, there has been an outpouring of individual
studies of biosorption, and there has been a seemingly almost random pairing between the
target pollutants and the nature of the biosorbent materials considered in different studies.
In view of the great amount of work already done, this appears to be a good time to
address some critical challenges that may be slowing down the pace and effectiveness of
implementation of bioremediation technologies at a commercial level.
The Diverse Nature of Natural Cellulosic Materials
A key challenge, which appears often to have been overlooked in the course of
studies of biosorption of pollutants, involves the huge diversity of naturally available
cellulosic materials, as well as their underutilized byproducts. Even before any steps
have been taken to modify the absorbent material, past studies have shown differences as
large as a factor of 1000 in the adsorption capacities for selected metals or dyes (Hubbe et
al. 2011, 2012). Given the existence of such huge ranges, one needs to be skeptical that a
given chemical or physical processing step can be justified in a given case. To give an
example, untreated kapok has been found to be a superb sorbent for spilled oils (Choi and
Cloud 1992; Lim and Huang 2007); in fact the performance of unmodified kapok for oil
sorption is already in a similar range to that of polypropylene fibers, which are the most
widely considered sorbent for such applications (Hubbe et al. 2013). Similarly, when
collecting cationic dyestuffs from water, various unmodified cellulosic materials meet or
exceed the performance of specialized sorbents, such as activated carbon products (Aksu
et al. 2010). Again there is a difference by a factor of about 1000 in adsorption capacity
between the most and least effective unmodified cellulosic material used from removal of
cationic dyes from water (Hubbe et al. 2012).
Various factors are likely to account for some of the great differences in
adsorption capacity obtained by different researchers carrying out nominally similar
studies. The accessible surface areas, per unit mass, might differ by large amounts,
depending on the conditions or preparation. The chemical nature of the surfaces may
differ substantially as well. In principle only a monolayer of material on a surface can
profoundly change its affinity for different kinds of sorbate molecules. For instance, the
presence of carboxylate species at the fiber surface is sometimes well correlated to the
uptake of cationic metal species from solution (Xei et al. 1996; Foglarova et al. 2009;
Klasson et al. 2009; Hubbe et al. 2011), and such a relationship can provide evidence of
an ion exchange mechanism (Gadd 2009). Such carboxylate groups might be present on
minor components such as certain hemicellulose species, or certain extractable fatty
acids, etc. Likewise, the lignin component of the substrate, to the extent that it is exposed
at the surface, might be important for sorption of sparingly soluble substances such as
dyes and oils (Lebek and Wardas 1996; da Silva et al. 2011; Kriaa et al. 2011).
Cellulosic material can be modified in many ways to improve its performance in
various applications. Some modifications that are most widely reported for increasing
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 389
the performance of biosorbents are aimed at increasing the amount of surface area that is
accessible by the target pollutant. One of the great challenges facing technologist
involves the large amounts of energy that often are required to open up cellulosic
materials and to make their interior surfaces more accessible. Take, for example, the
conversion of wood to chips. The amount of energy to achieve such a conversion is of
the order of 2 to 4.5 kJ/m3 (Lusth et al. 2013). If one takes the wood density to be
approximately 0.5 metric tons per m3, these values correspond to 1.3 to 2.5 X 10
-3 kWh/t.
Further reduction of biomass to powder form was estimated to require 47 to 91 kWh/t
(Sun et al. 2011), which is a huge amount of energy. Fortunately, a prudent technologist
or entrepreneur has the option of collecting sawdust as an underutilized byproduct from a
sawmill. Another approach is to use cellulosic materials that are inherently porous or
finely divided, e.g. the spent material from fermentation processes (Svecova et al. 2006;
Park et al. 2008a; Vijayaraghavan et al. 2008) or other microbial material (Ahluwalia and
Goyal 2005; Bishnoi and Garima 2005).
One of the simplest ways to change the surface nature of cellulosic materials is to
heat it up. Moderate heating (say up to 210 oC) can promote migration of low-surface-
energy material such as waxes to the accessible surfaces (Dundar et al. 2012; Kutnar et
al. 2013). Stronger heating in the absence of oxygen leads to torrefied products, for
which surface properties have been studied (Stelte et al. 2012). Finally, heating in the
range of 500 to 900 oC for controlled time periods in the absence of oxygen can convert
the material essentially to carbon. Post-treatment of the carbonized biomass, using KOH,
phosphoric acid, or steam, among other options, can greatly increase the internal
porosity; products made in this way are called “activated carbons” (Chowdhury et al.
2013). All of these heat-processed materials, if carefully optimized, can greatly increase
the uptake of various pollutants.
To complete the story, modifications involving chemical agents can change the
surface chemistry of sorbents in highly controllable ways. For example, reactions with
various anhydrides provide a convenient way of grafting various functional groups onto
the hydroxyl groups of cellulosic materials (Chandlia et al. 2009; Karnitz et al. 2010).
Also, cellulose-based surfaces can be carboxymethylated (Xie et al. 1996). By judicious
selection of the non-reactive end of the agent used for derivatization, it is possible to
achieve a wide range of charged character (including positive or negative charge) as well
as degree of hydrophobic or hydrophilic character.
Modeling and Quantification of Pollutant Uptake Increasing levels of sophistication have been reached by researchers in their use
of theoretical models to interpret the kinetic and equilibrium aspects of their biosorption
studies (Hubbe et al. 2011, 2012). The majority of the data sets considered in the cited
articles have been found to fit adequately to a Langmuir adsorption isotherm (Langmuir
1918), which assumes a fixed number of equivalent adsorption sites. Many authors have
explored alternatively models. The fact that certain data sets have been better fitted to the
Freundlich equation (Freundlich 1907) can be taken as evidence of heterogeneity of
adsorption sites, at least in those cases. Some authors have drawn mechanistic inferences
based on fits of adsorption isotherms using models such as the Redlich-Peterson Isotherm
(Padmesha et al. 2006), or multiple-species versions of the Freundlich or Langmuir
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 390
models (Fritz and Schlünder 1974; Aksu et al. 1997). Such attempts need to be viewed
with caution. In many such studies the data were found to simultaneously achieve good
fits (R2 >> 0.9) to more than one theoretical model (Hubbe et al. 2011), and the default
interpretations from the different models can be quite different from each other. This
type of problem in interpretation deserves careful study, possibly using new experimental
approaches to shed fresh light on the underlying mechanisms.
Similar challenges, involving competing possible interpretations, face those
investigators working to better understand the rates of uptake of pollutants onto cellulose-
based sorbents. The most widely used models for fitting kinetic data from such
experiments are a first-order diffusion model (Lagergren 1898), a pseudo-second-order
model (McKay and Ho 1999; Ho et al. 2000), and an inter-particle diffusion model
(Weber and Morris 1963). Of these three, a surprisingly large proportion of data sets
appear to be best described by the pseudo-second-order model. There seems to be no
consensus among researchers regarding what the good fits of data to the pseudo-second-
order model imply about the underlying mechanism. The form of the equation would
seem to imply that pairs of groups bound to the substrate need to simultaneously and
independently diffuse and collide with the adsorbate species in the rate-controlling step
(Hubbe et al. 2012). The literature appears to be silent regarding how such a mechanism
could possibly be to a valid description of what happens at fixed sites on a solid substrate.
Fortunately, there is a quite different way to explain why a pseudo-second order model
should be so successful in modeling the sorption isotherms of various heavy metals and
dyes. The form of the equation is consistent with a sharply decreasing opportunity for the
last available adsorption sites to be filled as the sorbent reaches saturation. Thus, the
model is a good one for cases in which the last-to-be filled sites are increasingly less well
suited for the adsorbate, either due to spatial constraints or due to an accumulation of
excess electrostatic charge due to adsorption of other ionic adsorbate molecules.
Another issue that seems to have received insufficient attention is the
effectiveness of different sorbents to reduce levels of water-borne pollutants to extremely
low levels. Ideally one would prefer sorbents having a favorable combination of high
sorption capacity and high affinity, allowing the residual concentration in the solution to
be kept at a very low level even after quite a high amount of the pollutant has been
adsorbed. One of the key concerns, in this regard, is that typical adsorbents are likely to
have a wide variety of surface sites – some offering high affinity for the pollutant, and
some offering lower affinity. The higher-affinity sites would be the most likely to
become filled relatively early in the process of biosorption. Because the remaining sites
would be lower in affinity, one would expect there to be a higher bulk concentration of
pollutant once the system reaches equilibrium. Though certain researchers have
attempted to model such situations, for instance by use of a multiple Langmuir model
with several classes of sites (Sheng et al. 2007; Hubbe et al. 2011), there is clearly a lot
more to be done from a practical perspective.
Certain potentially favorable strategies for water treatment deserve to be
evaluated at the lab scale. For instance, one can envision a system in which two packed
beds of adsorbent material are used in series. Presumably the first bed would be selected
having a high capacity for adsorption. The second one would be selected based on
having a very high affinity for the pollutant and an ability to decrease the bulk
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 391
concentration almost to zero. Such systems could be compared in the lab to either type of
bed by itself or to mixed beds in which the two sorbent materials are used together.
Some of the equations that would be needed to interpret results of such work already
have been demonstrated (Malkoc et al. 2006; Xiu and Li 2000).
Though quite many researchers have studied competitive adsorption among two
or more dissolved species, most such work has involved competition between pollutants
quite similar to each other. For instance, the following cited studies evaluated the
adsorption of mixtures of different cationic metal ions onto cellulose-based sorbents (Ho
and McKay 1999; Leyva-Ramos et al 2001; Aksu and Donmez 2006; Srivastava et al.
2006). Likewise, the following studies evaluated simultaneous removal of pairs of
relatively similar dyestuffs from a mixed solution (Porter 1993; Al-Degs et al. 2007). An
inherently more challenging situation is when contaminated water contains, say, a
cationic metal species (e.g. Pb(II) or Ni(II) ) and an anionic species, such as the chromate
ion (Aksu et al. 2002). The cationic species are often removed most effectively at a near-
neutral pH, whereas pH values of 1 to 3 are often required in order to achieve effective
adsorption of the anionic chromate (Hubbe et al. 2011). Further studies need to address
practical concerns for the simultaneous or sequential removal of multiple species, as
would be expected in the outfalls of many industrial processes.
Environmental Costs of Cellulose Modifications Each time that a natural material needs to be modified, in preparation for a
specified use, one can expect there to be an increase in the overall cost. Also, the amount
of energy expended and other adverse environmental consequences will tend to increase.
For these reasons, one of the major challenges that faces both researchers and policy-
makers involves how to quantify and minimize net environmental impacts. Some notable
recent progress can be cited (de Hoces et al. 2011). Some of the main strategies that can
be used to minimize greenhouse gas emissions and other adverse environmental impacts
related to bioremediation include the following:
Local sourcing of materials, thus minimizing the costs and energy requirements associated with transporting the sorbent
Bypassing or minimization of non-essential processing steps that use non-renewable energy
Avoidance of procedures that require use of solvents, especially if the solvents are not efficiently recycled or transformed into energy during the process
Avoidance of such landfilling practices that are likely to result in generation of methane due to the anaerobic decomposition of spent absorbent material
Avoidance of unnecessary drying steps may be considered to be one of the most
important challenges that will required research attention in the coming years.
Tabulation of experimental conditions used in studies involving the biosorption of metals
and dyes (Hubbe et al. 2011, 2012) has revealed surprisingly strong preferences among
researchers to thoroughly dry their sorbent material before its use. Such a preference is
probably motivated by concerns about the storage stability of wet or damp cellulosic
material. Also, a researcher would want to standardize the starting material in some way.
But drying requires a high energy input. It also entails risk of closing up pores within
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 392
the material, thus reducing the accessible surface area that is available for adsorption of
the pollutants (Stone and Scallan 1966).
The way that pore closure comes about during a typical drying process is
illustrated in Fig. 2. The figure envisions a slit-like pore within the material before
drying. As shown, during the evaporation of water from cellulosic material one can
expect there to be a meniscus formed between adjacent cellulosic surfaces.
R2
R1Pore within cell wall
Meniscus drawing
surface together
during drying
R2, the larger radius, fixes the meniscus area.
R1, the smaller radius, dominates the P term.
Fig. 2. Schematic representation of a meniscus of water tending to draw surfaces together within a slit-like pore of the cellulosic material during drying
Because of the surface tension at the air-water interface, and due to the curvatures
of the liquid surfaces, the pressure inside of the film of water can be much lower than the
ambient pressure. In other words, a tension or “vacuum effect” exists within the liquid,
drawing the two surfaces together. The maximum pressure difference can be estimated
from the Young-LaPlace equation (Page 1993; Chen et al. 2006),
P = 4 cos (1/R1 + 1/R2) (1)
where is the air-water interfacial tension, is the contact angle as measured through the liquid phase, R1 is the smaller radius of the meniscus, and R2 is the larger radius. After
making some simplifying assumptions, such as setting the contact angle to zero (to
represent the case of perfect wetting), Eq. 1 can be approximated as,
P 2 / x (2) where x is the thickness of the film of water. Since x is in the denominator of Eq. 2, the
predicted pressure difference tends toward infinity as x tends toward zero. Though the
assumptions underlying the use of Eq. 2 are sure to break down long before an infinite
force is reached, the existence of extremely strong forces has been shown by structural
damage to surfaces that have been dried in contact (Stratton and Colson 1993).
Another practice that needs to be re-examined is the grinding of cellulose-based
material before its use as a sorbent. Grinding requires energy. In addition, grinding
converts the material into fine particle that may slow down the flow through a packed bed
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 393
that is being used to remove pollutants from water. Alternatively, if the absorbent is
mixed into the polluted water as a suspension, then the finely suspended particles may be
more difficult to remove from the liquid by settling or filtration. The reason why
researchers have preferred the use of ground materials may be due to concerns about the
uniformity of samples, especially when the scale of laboratory experimentation may be
small. Also, it is logical for a researcher to expect that grinding would increase the
external surface area, thus increasing the sites available for adsorption. The problem with
using such logic is that the most effective sorbent materials tend to be porous, and
adsorption may take place mainly on internal surfaces – not just on the outside. There is
an urgent need for research dealing with the practical optimization of particle size with
respect to flow rates, separation steps, and also with the final processing of biosorbents
after they have been used.
What to Do with Used, Contaminated Sorbent Material One of the least-resolved aspects of biosorption involves the question of what to
do with the cellulose-based sorbent material after it has been used. An effort was made
to discover any articles dealing with this issue during preparation of recent review articles
(Hubbe et al. 2011, 2012, 2013). Only about 20% of the reviewed articles, dealing with
the topic of bioremediation, paid any attention at all to the subsequent fate or handling of
the sorbent material after its use. Of that 20%, most merely reported on the reversibility
of sorption when changing the pH conditions or some other treatment to reduce the
affinity of the substrate for the adsorbate.
Landfilling, i.e. the placement of waste materials in the ground, has been regarded
as the most likely destination of various contaminated materials (Carro et al. 2008).
Motivations for landfilling of spent sorbent materials could include relatively low cost
and expediency. Unfortunately, landfilling happens to be poorly suited to the disposal of
cellulose-based materials, which can be subject either to anaerobic decomposition or
leaching. The generation of greenhouse gases such as methane can be expected when
cellulose and related materials are kept in a moist, non-aerated environment (Padgett
2009). In terms of global warming, methane gas is about 21 times as harmful as carbon
dioxide (EPA 2013), though the predicted effects have been disputed due to differences
in the persistence of different gases in the atmosphere (Shoemaker and Schrag 2013).
While state-of-the-art landfill facilities attempt to collect such gases (Kumar et al. 2011),
there is a wide range in practices and equipment by which such systems are implemented.
Reversibility of sorption – for what purpose?
In principle, the “regeneration” of a material, allowing it to be used multiple times
for the same purpose, would tend to be considered as a positive attribute of the process as
a whole. However, each such application needs to be considered from a viewpoint of
minimizing the net adverse environmental impact. Suppose, for instance, that a
cellulose-based substrate is immersed in a strongly acidic solution to bring about the
release of heavy metal ions such as Pb(II) (see for instance Martínez et al. 2006). Under
such conditions, all of the carboxylic groups on the substrate become protonated, causing
cationic adsorbates such as Pb(II) become displaced back into the solution. In favorable
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 394
cases the sorbate can be restored nearly to its uncontaminated state, and its sorption
capacity can be almost completely restored as well.
Not a single author, out of all of the hundreds of articles considered in the series
of three review articles (Hubbe 2011, 2012, 2013) attempted to explain and justify the
logic of why it could make sense to collect metals, dyes, or oil from water and then to
turn around and change the conditions to put those same materials back into water again.
Presumably, by optimization of procedures, it may be possible to achieve a much higher
concentration of metals or dyes, etc., compared to the original effluent to which the
sorbent was applied. And maybe, despite a lack of supporting documentation in the
published literature, there are ways to extract value from such relatively more
concentrated solutions of metals, dyes, or oils. Readers who have an appetite for working
with toxic mixtures, especially if they can find ways to extract valuable components from
such mixtures, are likely to find relatively little competition from other researchers. The
challenge that one is likely to face, in addition to such factors as toxicity, extremes of pH,
and still rather dilute solutions, is that one can ordinarily expect to be dealing with
variable mixtures. In the case of mixtures of dyes, it may be difficult indeed to propose
any practical usage of the recovered material.
Since the adsorption step is key to the process as a whole, the following three
sections will deal with adsorption of metal ions, dyes, and oils, respectively. Each of
these sections will focus on how various modifications of cellulose-based sorbents have
been found to affect uptake of pollutants in each of the three cases. Subsequent sections
will deal with incorporation of biosorption into a practical cradle-to-grave scenario in
which the fate and processing of each component needs to be considered.
BIOSORPTION OF METALS, DYES, AND SPILL OIL
Metals Uptake and Effects of Cellulose Modifications As discussed at greater length elsewhere, one of the most promising situations for
use of cellulose-based sorbents can involve a polishing treatment to remove heavy metal
ions from dilute aqueous solutions (Hubbe et al. 2011). Other strategies, such as
chemical precipitation, distillation, and reverse osmosis (Fu and Wang 2011; Li et al.
2012; Plappally and Lienhard 2012), can make sense when dealing with relatively
concentrated solutions of metals, either to minimize the volume of waste material or to
convert it to a solid, more easily storable form. Relative to the other approaches listed,
biosorption appears to be well suited to working with highly dilute solutions. Two
situations likely to be especially promising for potential applications are (a) the final
treatment of wastewater before its release (after primary clarification to removal solids
and activated sludge treatment to remove biological materials), and (b) treatment of fresh
water to remove traces of toxic metals. Regulations and health concerns can influence
the levels of metals deemed acceptable in each of these two cases.
Most systems involving biosorption of metal ions can be understood in terms of
the principles of ion exchange (Gadd 2009). In other words, one envisions each
adsorbing metal ion as displacing a number of other ions, such as sodium or hydrogen,
corresponding to the valence of the metal species. Ion exchange mechanisms are thus
REVIEW ARTICLE Lignocellulose
Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 395
able to explain such things as the tendency of adsorbed amounts to be constrained by a
saturation level. Also, an ion exchange mechanism is consistent with an ability to reverse
the process, thus regenerating the adsorption ability of the substrate, by addition of a high
concentration of the initially present ions, e.g. immersion in a salt brine. In the case of
positively charged metal ions, ion exchange can be used to explain why a reduction in pH
tends to displace such ions from the substrate. What happens is that the metal ions’ ability
to adsorb becomes overwhelmed by the much higher concentration of competing
monovalent H3O+ ions.
A complication arises when considering metal ionic species having negative
charge. The most widely studied and important example is the chromate ion, CrO42-
(Agrawal et al. 2006; Mohan and Pittman 2006). If one assumes that the chromate ion
remains in its anionic form throughout an adsorption process, it is then difficult to
understand why it would tend to adsorb onto cellulosic substrates, which generally have
net negative surface charge at pH values greater than about 2.5. In fact, the highest
efficiencies of adsorption of the chromate ion generally are observed at pH values in the
range of about 1 to 3 (Gupta et al. 1999; Malkoc and Nuhoglu 2007; Hubbe et al. 2011).
At such low pH values the carboxyl groups at a cellulosic surface are essentially all
protonated, and protonation of some hydroxyl groups at extremely low pH appears to be
part of the mechanism.
A further complication arises due to the fact that the chromate ion is a strong
oxidizing agent, and cellulosic surfaces are susceptible to oxidation. Studies have shown
convincingly that adsorption of the chromate ion often entails simultaneous oxidation of
such surface groups, leading to the formation and adsorption of the trivalent Cr+3
species,
for which the maximum adsorption capacity is at a higher pH (Park et al. 2008a).
Because various carboxyl groups at cellulosic surfaces have pKa values in the
range of about 3.5 to 6, it makes sense to expect that the adsorption of cationic metal ions
ought to increase steadily with increasing pH. Many studies have shown such behavior
within acidic pH ranges (Hubbe et al. 2011a). However, a maximum uptake is often
observed somewhere within the pH range of 5 to 8, and beyond that maximum the uptake
falls steeply. The usual explanation is that multivalent metal cations can form neutral
hydroxide species at those higher pH values (Demirbas 2008). Despite the ubiquitous
nature of this phenomenon, there appears to have been very little research attention
devoted to isolation of such hydroxylated neutral metal species, possibly as a mechanism
for their removal from solution.
Many studies have explored effects of temperature on adsorption of metal ions
onto cellulose-based substrates. In cases where the adsorption decreases with increasing
temperature, such findings have been used as evidence for an exothermic nature of the
adsorption. In other words, heat is given off as the metal ions become adsorbed. But in a
great many cases, the situation has been found to be the reverse, with increasing
adsorption capacities in response to increasing temperature of equilibration. The latter
systems can be described as endothermic. In such cases the process still can occur
spontaneously due to the increasing freedom of motion experienced by the monovalent
ions released from the surface in the course of adsorption, i.e. due to increased entropy
(Rao et al. 2012).
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Hubbe (2013). “Review of cellulosic biosorption,” Lignocellulose 2(2), 386-411. 396
Because negatively charged surface groups at adsorbent surfaces clearly help in
the adsorption of cationic metal ions from aqueous solution, investigators have explored
many means of increasing the population of such groups (Hubbe et al. 2011). For
example, the amount of carboxylate groups at a cellulose-based surface often can be
increased by oxidation (Klasson et al. 2009), carboxymethylation (Xie et al. 1996), or
succinylation (Chandlia et al. 2009) reactions, among others. In such cases, large
increases in metal uptake have been documented (Hubbe et al. 2011). Even greater
effects have been achieved by grafting cellulosic surfaces with highly carboxylated
polymer chains (O’Connell et al. 2008). The cited results are consistent with the ion
exchange mechanism described earlier. In addition, the presence of multiple carboxylic
groups in close proximity to each other make it important to consider the involvement of
chemical complexation as an important contributing mechanism. In such cases, each
metal ion may be associated with more than one carboxylate group as a ligand. By
analogy to efficient chelating agents such as ethylenediaminetetraacetic acid (EDTA),
one can expect to achieve a very high affinity of cationic metal ions to such surfaces
(Gyurcsik and Nagy 2000). It follows that the use of highly carboxylated surfaces is a
useful strategy when the goal is to reduce the solution concentrations of metal ions to
very low levels.
Another important way to modify cellulosic materials in an attempt to improve
their ability to adsorb various pollutants is by heating them strongly. The results of such
treatment fall within a wide range depending on the intensity of the conditions used.
Relatively mild conditions of treatment, with temperatures up to 150 oC, are classified as
“drying”. The term “torrefaction” is used in an intermediate range of temperature,
usually with the exclusion of oxygen to avoid combustion (Chew and Doshi 2011).
When cellulosic materials are exposed to high temperatures, e.g. 400 oC to about 900
oC,
the process is called “pyrolysis” or “carbonation” (Chowdhury et al. 2013). As implied
by the latter term, the composition of the material becomes enriched in carbon, whereas
the amounts of hydrogen, oxygen, and traces of other elements tend to be reduced in the
carbonized material. To further enhance the ability of carbonized matter to adsorb
pollutants, the material is activated, a process in which the nanoscale porosity is
developed and optimized (Chowdhury et al. 2013). Activation involves strong heating in
the presence of such substances as phosphoric acid or KOH. Steam, carbon dioxide, and
various other treatments also can be incorporated into activation processes.
Although it can be said that high surface area tends to maximize adsorption
capacity for various pollutants, it can be equally important to speed up the rate with
which pollutant molecules can gain access to sites within an activated carbon material.
Thus, conditions of preparation (temperatures, times, concentrations of activating species,
etc.) are optimized to yield a favorable mixture of micropores (< 2 nm), mesopores (2 to
50 nm), and larger pores (> 50 nm) to facilitate relatively rapid diffusion of species into
the porous structure, while still providing the huge specific surface areas (e.g. as high as
about 1000 m2/g) associated with material incorporating the smallest micropores.
Though activated carbon species have been found to be highly effective for adsorption of
metal ions, it is important to bear in mind that similar levels of metal uptake have also
been achieved by use of other kinds of cellulose-based sorbent materials (Baillet et al.
1998; Loukidou et al. 2004; Hubbe et al. 2011).
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There has been a moderate level of research aimed at adjusting the surface
chemistry of cellulose-derived carbonaceous material for purposes of increasing the
adsorption of certain metal species (Lyubchik et al. 2004; Chowdhury et al. 2013). This
type of research is challenging due to the fact that any chemical change is likely to also
affect the distribution of pore sizes, surface areas, and other features of the material.
Also, attempts to introduce acidic groups onto the surface of activated carbon can be
expected to lead to oxidation of the material in general, even causing its combustion if the
conditions are pushed too far. While various results have been promising for enhancing
metal adsorption performance, this is an attractive area for future research.
Torrefaction deserves greater attention in future studies. The yield of torrefied
material from a given amount of biomass is much higher than the yield of activated
carbon. Though torrefied wood cannot be expected to allow development of nearly as
high specific surface areas as activated carbon products, the torrefied samples can be
expected to have a great diversity among their surface chemical sites. Torrefied products
also are more likely to retain pore structures associated with their origin as cellulosic
materials, and this may be potentially advantageous; however very little work of this type
has been reported.
Dye Uptake and Effects of Cellulose Modifications Water-soluble dyestuffs present in industrial streams and waterways are of
concern not only because of possible toxicity, but they are highly colored, difficult to
remove by conventional wastewater treatment, and often quite resistant to biodegradation
(Rai et al. 2005). Common dyestuffs tend to be somewhat hydrophobic, which can be a
great advantage when one wants to collect them onto a suitable absorbent from aqueous
solution. The aromatic groups and conjugated double bonded structures within industrial
dyes do not have a natural affinity for water, except that many dyes have been
synthesized with enough hydrophilic groups to permit their effective distribution onto the
material to be dyed. Another characteristic of the most common dyestuffs is that they are
predominantly organic materials, with only a small proportion of metal, if at all.
As in the case of adsorption of heavy metal ions, studies of dye uptake have
shown huge variations – even when the nominal conditions may be very similar (Hubbe
et al. 2012). It follows that there must be subtle differences in the way the experiments
were conducted. In particular, the cellulose-based substrates appear to have been handled
differently, perhaps even before their acquisition by the researchers. So, one challenge
for enterprising researchers in the future will be to gather fresh, never-dried materials
from the selected field, woods, or waste stream and to closely control each step during
storage, optional drying, and other processing steps.
One of the next challenges to consider, relative to biosorption, entails the diverse
nature of dyestuffs and the fact that an effluent stream may contain a mixture of them.
The most widely used dyes used in textile manufacture can be classed as reactive, direct,
acid, basic, or sulfur dyes, among others (Hunger 2003; Hubbe et al. 2012). Of these,
only the basic dyes have a positive charge, which provides a natural electrostatic
attraction onto the typically negatively charged surfaces of cellulose-based materials.
Many studies have addressed the relative efficiency of different sorbents to take up
methylene blue, which is a widely used basic dye (Peterlin et al. 2009; Acemioǧlu et al.
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2010; Aksu et al. 2010). Though such studies have revealed many interesting details
about the mechanism of dye uptake, the general consensus is that most ordinary cellulosic
materials can be used effectively for the removal of basic dyes from aqueous solution.
Though certain studies have demonstrated increased adsorption performance following
physical or chemical modification of the biosorbent (Gong et al. 2006; Chadlia and
Farouk 2007), the motivation for carrying out such modifications is not always clear.
Greater challenges can lie in the biosorption of negatively charged dye species
onto cellulosic materials. Modifications have been shown to be effective for improving
the uptake of selected anionic dyestuffs onto cellulose-based sorbents. These include
cationization (El Ghali et al. 2010), or pyrolysis to make activated carbon sorbents
(Thirumalisamy and Subbian 2010; Chowdhury et al. 2013). One way to account for the
positive effects of carbonation on dye uptake is to note that such treatments generally
increase the proportional aromatic content of the material and tend to make it more
hydrophobic. Also, the very high and optimized levels of surface area and pore sizes in
activated carbon products can lead to high adsorption capacity. Because many dye
species of interact are much larger than heavy metal ionic species, it may be
advantageous to adjust conditions of activation to yield larger pore sizes, i.e. more
mesopores relative to micropores.
Oil Uptake and Effects of Cellulose Modifications Oil spills are another major class of pollution that can be addressed, at least in
part, by use of cellulose-based absorbent materials. Oils, as a matter of definition, are
insoluble in water. A typical oil spill in water will include liquid oil, as well as possible
oil-in-water emulsions, water-in-oil emulsions, and minor amounts of dissolved
molecules (Hubbe et al. 2013). Here the main emphasis will be placed on removal of
liquid oil from water.
Because of the relatively hydrophilic character of cellulosic materials, key
challenges and opportunities can involve modifications to make the surfaces more
hydrophobic. Increased uptake of liquid oil from water onto such hydrophobically
modified sorbents has been shown (Sathasivam and Haris 2010; Gao et al. 2012; Payne et
al. 2012). Alternatively, high performance also has been achieved with certain cellulosic
materials that are naturally “waxy”, meaning that they tend to shed water. Examples
include kapok, milkweed seed hairs, and raw cotton fibers (Choi and Cloud 1992; Lim
and Huang 2007). The feasibility of any hydrophobic treatment strategies to make
cellulosic materials more suitable for oil sorption need to be weighed relative to these
naturally hydrophobic materials. One also can consider the use of high-performing
petrochemical-based sorbent materials, such as polypropylene fibers (Zahid et al. 1972;
Wei et al. 2003).
For use in water-free systems, the optimum surface character of an absorbent can
follow completely different rules. A recent study (Payne et al. 2012) showed that
hydrophobization of cellulose fibers can decrease the uptake of simulated crude oil in the
absence of water. Because the surface free energy of most oils is low, they will spread
spontaneously over higher-energy surfaces including those of cellulose-based materials.
In such cases, efforts to decrease the surface energy of the substrate surfaces, making
them more similar to that of the oil, may be unnecessary or even counterproductive.
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In submerged systems, or when the sorbent material may be subjected to wave
action, a hydrophobic character (or treatment) can render cellulose-based sorbents
substantially more effective for the collection of oils (Payne et al. 2012). In addition to
increasing the uptake of oil, the hydrophobic nature tends to hold onto bubbles, helping
the material to float at the water surface (Korhonen et al. 2011; Likon et al. 2011). The
possible sinking of sorbent material, either before it has become loaded with oil, is
regarded as highly undesirable (Fingas 2011).
A particular challenge in the sorption of liquid or emulsified oils can involve a
high viscosity of the pollutant. The high viscosity means that usually there will be little
advantage to be gained by having a high fine-scale content of porosity. Rather, the best
results are often achieved in such cases when using rather coarse, fibrous materials, in
which the oil may be accommodated with relatively large spaces between fibers (Ribeiro
et al. 2000). An exception to this rule is when one is interested in removing trace
amounts of dissolved oily pollutants from water; in such cases high-area sorbents such as
activated carbon, for instance, can be highly appropriate.
PROSPECTS FOR INTEGRATED APPROACHES TO BIOREMEDIATION
Looking towards the future, one can envision fully integrated biosorption systems
in which potential pollutants are efficiently removed from both effluent streams and from
the intake streams for potable water. The term “integrated” means that the removed
materials are collected and put to their most valuable use and that net environmental
impacts are minimized. The present section will focus on three such approaches and cite
various publications that touch upon the relative value of each of these general strategies.
Figure 3 shows schematic representations of the three main strategies to be considered.
Regeneration
scheme
Energy recoveryscheme
Biodegradation
scheme
Rinsate to waste treatment
Rinse solutionRegenerated sorbent to be used again
Drain Evaporate & burn
Energy
Disposal or
reclamation
Controlled
biodegradation
Composting Soil
Aerobic water treat. Sludge
Anaerobic w. t. Energy
Fig. 3. Schematic representations of three main strategies that could be used to deal with spent biosorbent material after its usage
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Collect, Regenerate (and concentrate), Reuse When an industrial material has been mixed with other materials and dispersed
into water that gets passed on to the environment, then it can be regarded as a pollutant.
But if the same material could be somehow concentrated and fractionated into its
relatively pure components, then it might be considered as being a valuable commodity.
It can be challenging, however, to put this principle into practice when dealing with
contaminated water. First, individual pollutants dispersed or dissolved into water can
have very low concentrations. It may be technologically challenging to come up with a
method that can gather up enough of the material into a sufficiently concentrated form to
have any commercial value. Furthermore, most polluted waters contain mixtures of
contaminants; this means that efforts at collection and concentration can be expected to
produce impure mixtures, which tend to have much lower value in comparison to pure
substances.
The collection of heavy metal ions from heavily contaminated water may
represent the best case scenario for efforts to reclaim a pollutant and isolate commercially
valuable materials (Cui and Zhang 2008). The metals industry has expertise in
concentrating and purifying various metals and related compounds (Tuncuk et al. 2012;
Coman et al. 2013; Xei et al. 2013). Furthermore, the metal content within spent
biosorbent material can be readily concentrated by the steps of drainage or pressing out
the water, drying the spent sorbent material, and incineration to obtain mineral-rich ash
(Gaballah and Kilbertus 1998). If each of the steps is done in an optimized manner, the
final ash should contain most of the metals originally collected onto the sorbent. In
principle, metals may be obtained by smelting or electrolytic reduction of oxides and
other compounds in the ash (Agrawal et al. 2009; Sheoran et al. 2009; Coman et al.
2013). Methods of separation of mixtures of metals can be challenging, but at least the
technologies are known.
As noted earlier, aqueous conditions can be adjusted such that classes of metal
ions are released back into the water phase (Hubbe et al. 2011). The resulting
regenerated sorbent material has been shown, in many cases, to be capable of adsorbing
almost the same amount of the selected pollutant when used for a second, third, or
additional time. This type of regeneration strategy seems especially appropriate for use
in flow-through systems, making it possible to regenerate a packed bed without having to
move the sorbent material (Chang 1997; Park et al. 2010). Ideally one can hope that the
liquid mixture created during the regeneration process has a much lower volume in
comparison to the effluent from which the heavy metal ions were initially collected. In
other words, one hopes to achieve a relatively high concentration of the metal ions in the
rinsate. Though many studies have specified conditions for effective regeneration
(Hubbe et al. 2011), the concentration of metal ions in such rinsate has hardly been
mentioned in such publications. Also, there has been almost no published comment
about what to do next with such rinsate. In theory it would be possible to employ a
variety of water treatment strategies to remove the metal ions from the water phase
(Lester 1987). But if that is the objective, then one has to wonder what was the point of
removing the metal species from the biosorbent in the previous step. It might make more
sense to use an inexpensive sorbent material a single time and then landfill it, allowing
the sorbent to continue its work of binding the metal species. More research is needed to
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evaluate the long-term stability of such binding under the conditions of landfilling
(Prechthai et al. 2008).
Dyes probably represent the worst-case scenario with respect to their collection
from polluted water and subsequent re-use. The human eye can be extremely effective in
discriminating minor differences in the coloration of manufactured objects. That means
that any commercial dyestuffs need to be prepared in a highly reproducible way. The
most successful strategies to redissolve dyes after their collection onto cellulosic material
involve adjustments in pH (Jain et al. 2010; Hubbe et al. 2012). However, such
approaches are typically only partially successful in releasing the dye into the solution
phase, and the approach makes sense only in the case of dyes that are strongly acidic (to
be released at high pH) or strongly basic (to be released at low pH). But that may be the
easy part; there is no practical way to physically separate similar dyes of differing hue
once they are present in the same aqueous mixture.
In the case of oils, one can envision a process analogous to dry cleaning in which
oils are extracted from spent biosorbent. Conventional dry cleaning is a costly process,
and additional costs would be added if efforts were made to remove the oil from the dry-
cleaning fluid. Perhaps the most promising systems that involve reclamation of oils from
sorbent materials are those that are incorporated into skimmer technology for collection
of recently-spilled petroleum from the water surface (Fingas 2011; Hubbe et al. 2013). In
such cases there can be sufficiently high amounts of oil that it can be squeezed from the
sorbent material (Choi et al. 1993; Silva-Tilak 2002). After separation of the oil and
water phases, the oil content may have commercial value.
Collect, Incinerate (and concentrate), Capture energy Various researchers have considered strategies in which the energy content of
biosorbent material is at least partly recovered after its usage (Chouchene et al. 2012;
Vanem et al. 2008). Such approaches can be justified, in many cases, by the fact that a
relatively large amount of absorbent material may have been used to collect minor
amounts of highly objectionable metals, dyes, or organic toxins from water. So rather
than to expect to recover value from the collected pollutants, one turns ones attention to
recovering value from the used absorbent.
One of the biggest challenges to recovering energy content from biosorbent
materials involves the water-soaked nature of many such materials. The energy required
to remove such water by some combination or draining, squeezing, and evaporation (with
emphasis on evaporation) has been considered in a few studies (Chouchene et al. 2012;
Jeguirim et al. 2012). One possible approach, which may minimize the amount of fuel
energy needed to dry biosorbent materials in preparation for their incineration, involves
spreading out the material to dry in the sunlight (Chouchene et al. 2012). This might be
an excellent approach for applications in hot, low-humidity climates, but perhaps not well
suited for north Mediterranean regions for which it has been initially investigated.
Another possible strategy would be to employ a multiple-effect evaporator system to
minimize the amount of steam that is needed to concentrate the material and get it ready
for final evaporation of the remaining water (Kumar et al. 2013). This type of system is
widely used for recovery of chemicals used in the pulping of wood for use in paper
manufacture.
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A particular challenge of schemes relying upon incineration is the tendency of
certain substances to either foul the incineration equipment or to raise the likelihood of
gaseous or airborne particulate pollution problems. As a large and visible source of
smoke and other toxic substances (Valerio 2010; Tsai and Wu 2013), incineration
operations are likely targets of regulation, fines, and litigation. If a sorbent has been
exposed to salt water during its use, perhaps due to collection of spilled oil from
seawater, then the salt not only can be corrosive to process equipment but also can
become part of the smoke (Viklund et al. 2013). Such problems can be overcome, for
instance by use of scrubber technologies (Le Cloirec 2012), and by advanced metallurgy
(Milos et al. 2012), but such fixes can be expensive.
Collect and Biodegrade A third main approach to dealing with used biosorbent materials involves
biodegradation. Though biodegradation can be regarded as being a natural process, the
conditions can be set up in such a way as to speed up the process and to minimize the
generation of unwanted gaseous species. Three major classes of such treatment are
composting, conventional secondary treatment of wastewater, and anaerobic digestion.
Though each of these kinds of treatment has been well studied, relatively little attention
has been paid to their applications to consume or decontaminate spent biosorbent
materials after their use.
Composting can be defined as a controlled process of aerobic decomposition of
damp material in which conditions allow for a substantial rise in temperature, thus
increasing the rate of biodegradation (Hubbe et al. 2010). By placing the degradable
material in a sufficiently large pile, the heat generated by microbial metabolism can be
sufficient to maintain the pile at temperatures that are ideal for various classes of
microbes. Enzymes released by fungal and bacterial species naturally present in the
materials can proliferate and help to break down the lignocellulosic matter. The goal of a
typical composting operation is to (a) eliminate any toxic, odorous, or disease-generating
substances, (b) reduce the amount of material, and (c) to end up with a humus-rich
mixture that can enhance the fertility and water-holding ability of soil. In particular,
break-down products of the slow-to-decompose lignin components of wood, i.e. the
humus, is known to hold strongly to both water molecules and essential minerals (Selim
and Zhang 2013).
The relationship between composting processes and polluting substances has
attracted relatively little attention from researchers (Semple et al. 2001; Purnomo et al.
2011). Perhaps this is because the incorporation of serious pollutants into a composting
process is viewed as being contrary to the goal of producing a high-quality amendment
for healthy soil. Also, there may be reluctance among some technologists to seriously
consider using a process that is more often associated with organic gardening. Since the
humus within compost tends to resist further aerobic breakdown (Hubbe et al. 2010), one
can expect it to remain in the soil (or landfill), binding heavy metals and/or hydrophobic
materials, for many decades into the future. However, research to back up such
expectations is needed. Key questions to be addressed in such research are as follows:
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Even before composting, is the pollutant already being held with sufficient permanency as to preclude significant toxic effects and to call into question the
need for the proposed composting treatment?
Can conventional composting processes reduce the toxic or other adverse environmental effects of the pollutants in addition to the known ability of
composting to minimize present and future emission of greenhouse gases from the
organic matter?
Does the compost resulting from processing of spent biosorbent need to be excluded from certain uses, such as the conditioning of soils to be used for the
growing of food crops?
Conventional secondary treatment of wastewater is similar to composting with
respect to its reliance on aerobic microbes – as well as the enzymes released from the
microbes – to break down various organic molecules. What is different is that everything
takes place under water during secondary wastewater treatment. Some of the biological
sludge that is generated during such processing, the so-called activated sludge, usually is
recirculated to the next incoming batch of wastewater to be treated (Ni and Yu 2012). In
that way the operators are able to inoculate each batch with a diverse population of
bacteria and fungi, allowing for the efficient breakdown of such species as hemicellulose,
starches, etc. But certain dye species are known to be resistant to biodegradation (Khan
et al. 2013). In some cases it is this very resistance to biodegradation that has prompted
technologists to consider biosorption in the first place. So it may in the end make more
sense to carry out the conventional wastewater treatment first and then to employ
biosorption as a follow-up, polishing treatment to reduce the remaining concentrations of
dissolved pollutants to very low levels.
Anaerobic digestion is the other main strategy that needs to be considered
seriously as a way of dealing with spent biosorbent. Such treatment can offer effective
detoxification of many toxic organic compounds (van der Zee and Villaverde 2005;
Hwang et al. 2007). By using anaerobic digestion it is also possible to collect flammable
byproduct gases such as hydrogen and methane, which then can be used in place of fossil
fuels. Such research could be extended to consider what happens when such pollutants
are to be treated while they remain adsorbed on the surface of a biosorbent, upon which
they have been collected. A question to figure out is whether the presence of cellulose-
based material during anaerobic treatment might either promote or interfere with the
desired chemical transformations.
CLOSING STATEMENTS
The adsorption of pollutants such as heavy metals, dyes, and spilled oils onto
cellulose-based materials appears to be a promising approach with good prospects for
more widespread usage in years ahead. But there are a great many questions that await
the attention of researchers. In particular, more work is needed to determine the most
appropriate way of dealing with such sorbent materials after the target pollutants have
been removed from the water. Modification of cellulose-based materials appears to be an
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effective strategy to improve their effectiveness for reducing water-borne pollutants to
very low concentrations. Each such modification needs to be scrutinized relative to the
net environmental impacts. Technologists and entrepreneurs need also to consider the
possibility that certain as-received cellulosic materials often can outperform various other
modified sorbents. In other cases, modest steps such as heat treatment or oxidation can
have potential to yield large benefits in terms of pollution control that more than
compensate for the environmental costs associated with the treatments themselves.
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